When reducing the resolution of an image, the program basically analize the color date of each pixel and get rid of a lot of pixel in order to make the image smaller. it doesn't keep the information about the lost pixels anywhere. So there really isn't a way to go back to what you had.
You can, however, increase the size and resolution (number of pixels per inch) it won't give you a result that's as good as the original, but it will be larger.
The best way to do it is, instead of increasing the image to the size you want it to be all at once, is to increase it size to 110% (on image size) and then repeat the process till the image is the size you want it to be, or until it start to get blurry or pixelate (whatever comes first).
It depends on the image and it's size, but in some images I manage to do that 6 or 7 times and the result was pretty good.